High density data storage is in great demand, and holographic techniques enable storing data at higher density relative to other data storage techniques. Holographic data storage also facilitates faster transfer rate for recording and retrieving data compared to other optical storage techniques and to magnetic tape based data storage.
Holographic data storage generally includes modulating a light beam to contain data, with the modulated light beam often referred to as a signal beam. The light beam is typically modulated to contain an image of a two dimensional pattern of light and dark pixels. An interference pattern created between the signal beam and a reference beam is typically recorded as a hologram in a three dimensional volume of photosensitive storage medium. The hologram comprises a diffraction grating from which a duplicate of the signal beam, containing a duplicate of the pixel pattern image, can be generated by use of a probe beam familiar to persons skilled in the art. The probe beam is generally identical or very similar to the reference beam (or its conjugate) used to record the hologram. The duplicate of the signal beam can be referred to as a reconstructed signal beam, and the duplicate of the pixel pattern image can be referred to as a reconstructed image or a holographic image of the pixel pattern.
Recording the hologram in three dimensions facilitates using an interior volume of the photosensitive storage medium for data storage. In contrast, non-holographic optical data storage techniques usually store data in a two dimensional area residing on a surface of the storage media. Variations in which multiple layers of stored data reside beneath a storage media surface are known, but light scatter caused by the multiple layers usually limits the extent to which multilayer techniques can be exploited to increase storage density in non-holographic optical data storage.
Holographic data storage can include techniques for recording multiple holograms, each of which includes an individual pixel pattern, in a common volume of storage medium, such that the stored multiple holograms (i.e. multiple diffraction gratings) at least partially overlap with each other in the common volume. Such techniques are commonly referred to as multiplexing or multiplex holography, and the at least partially overlapping holograms can be referred to as being multiplexed. Individual holographic images can be reconstructed from the multiplexed holograms by use of appropriate reconstruction techniques familiar to persons skilled in the art. Known multiplexing techniques include, but are not limited to, angle, wavelength, collinear, and polytopic multiplexing.
Different multiplexing techniques can be combined to increase data storage density. For example, a stack of angle multiplexed holograms can be stored partially overlapping an adjacent stack of angle multiplexed holograms, but offset from completely overlapping by at least a beam waist. So configured, the holograms residing in the stack and the adjacent stack can be referred to as being polytopically multiplexed.
Data storage requirements appear to be ever increasing, and increased data storage density is thus in demand. Accordingly, new techniques that increase density at which data can be stored holographically are needed. Additional benefit may reside where the new techniques can be combined with known multiplexing methods.